It is well known that DFB semiconductor lasers can have many advantageous features (e.g., extremely narrow-band emission) that make them desirable for, inter alia, optical fiber communications. However, there generally are at present two major technological problems in the manufacture of such lasers. One is the reproducible control of the optical feedback coupling coefficient .kappa., and the other is the discrimination of one definite oscillation wavelength out of the two possible oscillations at the edges of the Bragg reflection band.
The former problem is known to have a very significant effect on laser characteristics, e.g., spectral linewidth, harmonic distortion, and intensity noise, and thus can seriously affect manufacturing yield. The latter, if uncontrolled, can substantially reduce the fraction of devices having a specified wavelength, and thus also affects manufacturing yield.
One known approach to addressing the oscillation wavelength degeneracy problem in index-coupled DFB lasers is the use of antireflection/high reflectivity (AR/HR)-coated facets. This, however, can also cause a yield problem due to the phase uncertainty at the facets. Another approach is the incorporation of a .lambda./4 or corrugation pitch modulated phase shift. For perfect AR coatings, DFB lasers could in principle be produced with high yield, but the yield deteriorates rapidly for (typically encountered) reflectivities of a few percent. Furthermore, such lasers waste substantially half of the power by emission through the back facet, and can exhibit high spatial hole burning, which typically gives rise to optical non-linearity in the light-current curves, increased spectral linewidth, and a less flat frequency modulation response.
An alternative approach to the wavelength degeneracy problem is the introduction of gain coupling. See H. Kogelnik et al., Journal of Applied Physics, Vol. 43, p. 2327 (1972). Theory predicts that a purely gain coupled laser should have one lasing mode exactly at the Bragg wavelength for AR-coated facets (thereby solving the degeneracy problem), and that even a small degree of gain coupling can be advantageous for both AR-coated and non-AR-coated lasers. The validity of the gain-coupled approach has recently been demonstrated in GaAs/AlGaAs DFE lasers. See Y. Luo et al., Applied Physics Letters, Vol. 56(17), p. 1620 (1990); Y. Luo et al., IEEE Journal of Quantum Electronics, Vol. 27(6), p. 1724 (1991). These papers disclose a DFB laser comprising an active layer of periodically varying thickness. This is achieved through etching of a conventional grating of period .LAMBDA. into the surface of an appropriate multi-layer semiconductor body, growing a buffer layer over the grating such that the exposed buffer layer surface is corrugated (with period .LAMBDA.), and growing the active layer on the exposed corrugated buffer layer surface such that the active layer surface is flat. This clearly is a complicated procedure that cannot easily be incorporated into a manufacturing process. See also T. Inoue et al., IEEE Transactions Photonics Technology Letters, Vol. 3(11), p. 958 (1991), which also discloses a gain coupled DFB laser with a corrugated active layer.
A different approach is disclosed in B. Borchert et al., IEEE Transactions Photonics Technology Letters, Vol. 3(11), p. 955 (1991) and Y. Nakano et al., Applied Physics Letters, Vol. 55(16), p. 1606 (1989). Both papers disclose gain-coupled DFB lasers comprising a periodic loss structure that is spaced apart from the active region of the laser.
Although there are now known techniques that address at least one of the above referred-to two problems, the techniques generally are complicated and/or not fully effective. For instance, the gain-coupled DFB lasers of the two last cited papers have relatively complex structures that address only the mode degeneracy problem but do not address the coupling constant control problem.
In view of the importance of increasing the manufacturing yield of acceptable DFB lasers it would be very desirable to have available a simple laser design that can reliably overcome the above referred-to two problems. This application discloses such a design.